The influence of humic acid on the pH-dependent sorption of americium(III) onto kaolinite

This work investigates the sorption of ameri- cium (Am(III)) onto kaolinite and the influence of humic acid (HA) as a function of pH (3-11). It has been studied by batch experiments (V/m = 250:1 mL/g, CAm(III) = 1 9 10 -5 mol/L, CHA = 50 mg/L). Results showed that the Am(III) sorption onto the kaolinite in the absence of HA was typical, showing increases with pH and a distinct adsorption edge at pH 3-5. However in the presence of HA, Am sorption to kaolinite was significantly affected. HA was shown to enhance Am sorption in the acidic pH range (pH 3-4) due to the formation of additional binding sites for Am coming from HA adsorbed onto kaolinite surface, but reduce Am sorption in the intermediate and high pH above 6 due to the formation of aqueous Am- humate complexes. The results on the ternary interaction of kaolinite-Am-HA are compared with those on the binary system of kaolinite-HA and kaolinite-Am and adsorption mechanism with pH are discussed. Effect of different molecular weight of HA, with three HA fractions separated by ultrafiltration techniques, on the Am sorption to kao- linite were also studied. The results showed that the enhancement of the sorption of Am onto kaolinite at the acidic pH conditions (pH 3-4) was higher with HA frac- tions of higher molecular weight. Also, the Am sorption over a pH range from 6 to 10 decreased with decreasing molecular weight of HA.